[gcc r11-2457] Set and test no-warning bit to avoid -Wnonnull for synthesized expressions.

[Martin Sebor]

https://gcc.gnu.org/g:df5cf47a978aaeb53fc2b18ff0b22eb4531a27d8

commit r11-2457-gdf5cf47a978aaeb53fc2b18ff0b22eb4531a27d8
Author: Martin Sebor <msebor@redhat.com>
Date:   Fri Jul 31 10:27:33 2020 -0600

    Set and test no-warning bit to avoid -Wnonnull for synthesized expressions.
    
    Resolves:
    PR c++/96003 spurious -Wnonnull calling a member on the result of static_cast
    
    gcc/c-family/ChangeLog:
    
            PR c++/96003
            * c-common.c (check_function_arguments_recurse): Return early when
            no-warning bit is set.
    
    gcc/cp/ChangeLog:
    
            PR c++/96003
            * class.c (build_base_path): Set no-warning bit on the synthesized
            conditional expression in static_cast.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/96003
            * g++.dg/warn/Wnonnull7.C: New test.

Diff:
---
 gcc/c-family/c-common.c               |  3 +++
 gcc/cp/class.c                        | 10 ++++++++--
 gcc/testsuite/g++.dg/warn/Wnonnull7.C | 36 +++++++++++++++++++++++++++++++++++
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index b97539c0c2a..96ed2334863 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -5822,6 +5822,9 @@ check_function_arguments_recurse (void (*callback)
 				  void *ctx, tree param,
 				  unsigned HOST_WIDE_INT param_num)
 {
+  if (TREE_NO_WARNING (param))
+    return;
+
   if (CONVERT_EXPR_P (param)
       && (TYPE_PRECISION (TREE_TYPE (param))
 	  == TYPE_PRECISION (TREE_TYPE (TREE_OPERAND (param, 0)))))
diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index 7a25d8fc76c..b39bdaaa3ab 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -516,8 +516,14 @@ build_base_path (enum tree_code code,
 
  out:
   if (null_test)
-    expr = fold_build3_loc (input_location, COND_EXPR, target_type, null_test, expr,
-			    build_zero_cst (target_type));
+    {
+      expr = fold_build3_loc (input_location, COND_EXPR, target_type, null_test,
+			      expr, build_zero_cst (target_type));
+      /* Avoid warning for the whole conditional expression (in addition
+	 to NULL_TEST itself -- see above) in case the result is used in
+	 a nonnull context that the front end -Wnonnull checks.  */
+      TREE_NO_WARNING (expr) = 1;
+    }
 
   return expr;
 }
diff --git a/gcc/testsuite/g++.dg/warn/Wnonnull7.C b/gcc/testsuite/g++.dg/warn/Wnonnull7.C
new file mode 100644
index 00000000000..7611c18d948
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wnonnull7.C
@@ -0,0 +1,36 @@
+/* PR c++/96003 - spurious -Wnonnull calling a member on the result
+   of static_cast
+   { dg-do compile }
+   { dg-options "-Wall" } */
+
+struct D;
+struct B
+{
+  B* next;
+  D* Next ();
+};
+
+struct D: B
+{
+  virtual ~D ();
+};
+
+struct Iterator
+{
+  D* p;
+  void advance ()
+  {
+    p = static_cast<B*>(p)->Next ();    // { dg-bogus "\\\[-Wnonnull" }
+  }
+};
+
+// Test case from comment #11.
+
+struct S1 { virtual ~S1 (); };
+struct S2 { virtual ~S2 (); };
+struct S3: S1, S2 { void f (); };
+
+void f (S2 *p)
+{
+  static_cast<S3 *>(p)->f ();           // { dg-bogus "\\\[-Wnonnull" }
+}